The present invention relates generally to microwave oven cavity excitation systems and, more particularly, to microwave oven cavity excitation systems for promoting time-averaged uniformity of microwave energy distribution within the cooking cavity, and for improving the impedance matching of the microwave energy generator to the load by reducing the amount of reflected energy returned to the generator.
In a microwave oven cooking cavity, the spatial distribution of the microwave energy tends to be non-uniform. As a result, "hot spots" and "cold spots" are produced at different locations. For many types of foods, cooking results are unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed. The problem becomes more severe with foods of low thermal conductivity which do not readily conduct heat from the areas which are heated by the microwave energy to those areas which are not. An example of a food falling within this class is cake. However, other foods frequently cooked in microwave ovens, such as meat, also produce unsatisfactory cooking results if the distribution of microwave energy within the oven cavity is not uniform.
A conventionally accepted explanation for the non-uniform cooking pattern is that electromagnetic standing wave patterns, known as "modes," are set up within the cooking cavity. When a standing wave pattern is set up, the intensities of the electric and magnetic fields vary greatly with position. The precise configuration of the standing wave or mode pattern is dependent at least upon the frequency of microwave energy used to excite the cavity and upon the dimensions of the cavity itself. (While it is possible to theoretically predict the particular mode patterns which may be present in the cavity, it should be noted that actual experimental results are not always consistent with theory).
In an effort to alleviate the problem of non-uniform energy distribution, a great many approaches have been tried. The most common approach is the use of a device known as a "mode stirrer," which typically resembles a fan having metal blades. The mode stirrer rotates and may be placed either within the cooking cavity itself (usually protected by a cover constructed of a material transparent to microwaves) or, to conserve space within the cooking cavity, may be mounted within a recess formed in one of the cooking cavity walls, normally the top.
The function of the mode stirrer is to continually alter the mode pattern within the cooking cavity. If a particular mode exists for only a moment, and then is immediately replaced by a mode having different hot and cold spots, then, averaged over a period of time, the energy distribution within the cavity is more uniform. In addition to varying reflection properties, a mode stirrer also tends to "pull" the oscillation frequency of the magnetron (which is a self-oscillating device) about the 2450 MHz center frequency. The cyclical variation in precise operation frequency causes different modes to be theoretically possible in the oven cooking cavity, depending also upon the precise cavity dimensions.
A variation on the mode stirrer approach is to employ a dual feed arrangement with separate mode stirrers or other forms of moving reflector bodies to vary the manner in which microwave energy is introduced into the cavity, with the object generally being to introduce further randomness in the resultant field pattern within the oven cavity, and to even out the energy distribution on a time-averaged basis. By way of example, the following U.S. patents are identified for their disclosures of various dual-feed microwave oven excitation systems: Reftmark's U.S. Pat. No. 3,364,332; Cougoule's U.S. Pat. No. 3,439,143; Wikstrom et al U.S. Pat. No. 3,742,177; Imberg et al U.S. Pat. No. 3,993,886; Thuleen's U.S. Pat. No. 4,133,997; and Baron et al U.S. Pat. No. 4,140,888.
In various refinements to the dual feed approach, individual known fields from a plurality of feed points are combined in a definite manner to predictably produce a resultant field within the microwave oven cavity. The following U.S. patents are identified for their disclosures of this general type of system: White U.S. Pat. No. 3,739,130; Couasnard's U.S. Pat. No. 3,745,292; and Kaneko et al U.S. Pat. No. 4,176,366. A somewhat related disclosure may be found in the commonly-assigned Hauck U.S. Pat. No. 4,144,436, wherein energy is coupled into the cooking cavity through a single cross-shaped opening with two sets of E fields being produced oriented at right angles to each other.
Another consideration in microwave oven design is minimizing load mismatch presented to the microwave energy generator, typically a magnetron, or, stated alternatively, minimizing electromagnetic wave energy reflected back to the generator. This is an especially important consideration where the generator and cooking cavity are relatively closely coupled, and an extremely wide variety of loading conditions may exist in the oven cavity as foods of various quantities and types are placed therein. One prior art approach which, when suitably designed and adjusted, virtually eliminates energy reflected back to the generator, employs an isolator in the form of a directional coupler or microwave circulator to divert reflected wave energy to a microwave absorber, rather than allowing reflected microwave energy to return to the generator. One example of this approach is disclosed in the Nagai U.S. Pat. No. 3,437,777. Another such arrangement is disclosed in the above-referenced Couasnard U.S. Pat. No. 3,745,292. While such arrangements are indeed effective for the purpose of preventing microwave energy generator load mismatch, they introduce complexity and attendant cost into the microwave oven, and additionally are not as efficient because a certain amount of energy is necessarily and deliberately dissipated in a microwave absorber, where the energy is simply converted to heat and not effectively transmitted to the food load being cooked.
From the foregoing brief summary of a variety of approaches to achieving time-averaged uniformity of energy distribution, it will be appreciated that this remains a formidable consideration in the development of practical microwave ovens.
The present invention provides a microwave energy excitation system which advantageously promotes time-averaged uniformity of microwave energy distribution within the cooking cavity and, at the same time, reduces the amount of reflected energy which reaches the microwave energy generator by redirecting this energy towards the food load, rather than by directing it to an absorber.